High-power laser systems are utilized for a host of different applications, such as welding, cutting, drilling, and materials processing. Such laser systems typically include a laser emitter, the laser light from which is coupled into an optical fiber (or simply a “fiber”), and an optical system that focuses the laser light from the fiber onto the workpiece to be processed. The optical system is typically engineered to produce the highest-quality laser beam, or, equivalently, the beam with the lowest beam parameter product (BPP). The BPP is the product of the laser beam's divergence angle (half-angle) and the radius of the beam at its narrowest point (i.e., the beam waist, the minimum spot size). The BPP quantifies the quality of the laser beam and how well it can be focused to a small spot, and is typically expressed in units of millimeter-milliradians (mm-mrad). A Gaussian beam has the lowest possible BPP, given by the wavelength of the laser light divided by pi. The ratio of the BPP of an actual beam to that of an ideal Gaussian beam at the same wavelength is denoted M2, or the “beam quality factor,” which is a wavelength-independent measure of beam quality, with the “best” quality corresponding to the “lowest” beam quality factor of 1.
Wavelength beam combining (WBC) is a technique for scaling the output power and brightness from laser diode bars, stacks of diode bars, or other lasers arranged in one- or two-dimensional array. WBC methods have been developed to combine beams along one or both dimensions of an array of emitters. Typical WBC systems include a plurality of emitters, such as one or more diode bars, that are combined using a dispersive element to form a multi-wavelength beam. Each emitter in the WBC system individually resonates, and is stabilized through wavelength-specific feedback from a common partially reflecting output coupler that is filtered by the dispersive element along a beam-combining dimension. Exemplary WBC systems are detailed in U.S. Pat. No. 6,192,062, filed on Feb. 4, 2000, U.S. Pat. No. 6,208,679, filed on Sep. 8, 1998, U.S. Pat. No. 8,670,180, filed on Aug. 25, 2011, and U.S. Pat. No. 8,559,107, filed on Mar. 7, 2011, the entire disclosure of each of which is incorporated by reference herein.
A variety of WBC techniques have been utilized to form high-power lasers for a host of different applications, and such techniques often involve the formation and manipulation of beams (including the multi-wavelength output beam) having fast and slow diverging axes. However, WBC systems may utilize optical pathways that are more than a meter long and that incorporate multiple optical elements (e.g., mirrors) therewithin. Variations in the optical beams of the system, particularly along the non-WBC direction (which is typically, but not necessarily, the slow axis for high-power WBC laser systems), may be introduced at each interface with an optical element, particularly by such phenomena as thermal lensing, heating of the system housing, vibration, or other shifts or misalignments of the optical elements. Compensation for such variations may be difficult, making it harder to maintain the resonator feedback from the output coupler at levels sufficient for lasing. Thus, there is a need for improved optical arrangements that correct such optical variations, thereby enabling improved WBC laser systems.